The input impedance and input bias are two important factors of the design of an amplifier. For instance, to amplify a signal from a high output impedance signal source, the input stage of the amplifier must have high input impedance to reduce the signal attenuation resulted from signal coupling loss. However, if an amplifying circuit with high amplification is employed in the input stage of the amplifier, the larger size of the circuit components would result in smaller input impedance, which in turn prevents the signal from the high output impedance signal source from being coupled to the amplifier completely, and consequently lead to signal attenuation. The conventional amplifiers often use bias circuits to provide the desired high input impedance thereof, though this gives rise to other problems. On the other hand, the output bias needed at the output of the signal source often differs from the input bias at the input stage of the amplifier, which hinders the direct connection between the amplifier and the signal source. If an amplifier configuration can allow adequate bias for the output of the signal source, the input bias adaptability of the amplifier can be enhanced and the amplifier can be suitable for various signal sources, thus widening the applications of the amplifier.
When designing the circuitry of an amplifier, factors such as cost and overall size of the circuit must be taken into account, and this is especially true if other factors like noise reduction may be overlooked, where the cost and the size of the amplifier are the most critical factors to consider. Provided that the performance of the amplifier is not overly compromised, reducing the size of the amplifier, such as integration into a single chip, and lowering the cost are the top priority for the designers of the amplifiers currently.
U.S. Pat. No. 3,595,998 proposed a preamplifier for microphones which uses a polarity-dependent bias circuit to control the gate voltage of the FET of an amplifier, and provides individual bias for the signal source of the amplifier. However, the polarity-dependent bias circuit is complex and huge and, as described in U.S. Pat. No. 6,812,788, it is required to have a resistance up to tens or even hundreds of GΩ and will induce severe noise problem. In addition, to provide so much resistance, external resistor is required, thereby causing that the circuitry cannot be miniaturized and has higher cost.
U.S. Pat. No. 5,337,011 proposed a preamplifier for microphones which uses two cascode stages to improve the impedance matching, in order to prevent severe gain loss and inhibit noises. Unfortunately, this circuit is also complex and never solves the adaptability problem of the input bias.
U.S. Pat. No. 7,110,560 proposed a preamplifier for microphones which uses a pair of cross-coupled diodes to provide high input impedance, and a coupling capacitor to prevent DC leakage. However, the cross-coupled diodes at the input will introduce other problems as described in U.S. Pat. Publication No. 20030194100, and further, this art still cannot solve the adaptability problem of the input bias.
U.S. Pat. Publication No. 20030194100 proposed an input buffer bias circuit for microphones which uses a current limiter to limit the current of the cross-coupled input bias diodes to increase the voltage level of the input signal. However, this art still do not solve the adaptability problem of the input bias.
U.S. Pat. No. 6,888,408 proposed a preamplifier for microphones which uses a two-stage amplifier to replace the conventional junction transistor (JFET), and in which the first stage amplifier minimizes the input capacitance, and the second stage amplifier optimizes the gain. However, the first stage amplifier must be designed to match the output capacitance of the signal source, and thus it is designed according to the signal sources of various output capacitances one by one, which not only restricts the applications of the amplifier but also increases the cost. In-addition, this art does not solve the adaptability problem of the input bias.
U.S. Pat. Publication No. 20050151589 proposed an amplifying circuit of a capacitive transducer, which also uses a pair of cross-coupled diodes to provide high input impedance, and a servo-amplifier to feed back the output to the pair of cross-coupled diodes in order to control the input bias point. However, the cross-coupled diodes at the input will induce other problems, and this art still does not solve the adaptability problem of the input bias.
U.S. Pat. No. 6,812,788 proposed an amplifying circuit for a capacitive microphone which uses independent bias power supplies to set the bias voltages of the signal source and the amplifier input respectively, and a network of diodes and resistors with high resistance to replace conventional coupling resistor with high resistance and feed back the output to the input coupling network. However, this art needs two bias power supplies and uses the input coupling network for providing high input impedance, resulting in increased complexity and cost of the amplifier circuit design.